Ceramic materials have been used extensively in fabricating integrated circuits and the like. Because of their low resistance to externally exerted forces, ceramics of poor quality tend to crack due to thermal or mechanical shock during soldering or machining operations. Since it is of the utmost importance that ceramic components be of high quality, a technique for non-destructively detecting the cracking of parts which may not be visually accessible during processing is highly desirable.
U.S. Pat. No. 3,924,456, which issued on Dec. 9, 1975 to S. J. Vahaviolos and is assigned to the instant assignee, is directed to detecting microcracks in ceramic substrates using SWE techniques. An SWE, also referred to as an Acoustic Emission, may be defined as an elastic wave propagated in a structure as the result of an applied stress and which is characterized by low amplitude, short duration and fast rise time. A stress wave event, caused by a crack, has an initial high pulse which is followed by a series of lower amplitude pulses having an exponentially decaying envelope. An SWE signal may comprise a multitude of such events.
The Vahaviolos patent describes a piezoelectric transducer mounted proximate the substrate, which is subjected to a load, to detect stress waves emitted from the microcracks as they form. A processing circuit generates an output signal indicative of a detected crack when the magnitude of the electrical output from the transducer exceeds a predetermined system noise level.
Such a technique has been found to be most effective when the system noise levels are low. However, at times, the ambient noise vibration due to automatic soldering tools, bonders or associated mechanical movements can have the same amplitude or even a greater amplitude than the SWE events. These mechanical vibrations translate into relatively high oscillating signals which can be incorrectly interpreted by the stress wave detection apparatus as being caused by a crack in the material, resulting in loss of acceptable product.
U.S. Pat. No. 4,086,816 which issued on May 2, 1978 to Jon et al. and is also assigned to the instant assignee is directed to distinguishing between SWE signals and noise signals. That patent teaches the counting of detected signal pulses passing through a threshold during a predetermined period of time while simultaneously counting the number of excursions of the envelope of the pulses passing through the threshold. A ratio of the signal pulse count to the envelope excursion count made during the same time period is formed and compared to an empirically determined range of ratios which are indicative of an SWE signal.
The technique disclosed in the Jon et al. patent has been most successful. However, there are instances where certain noise signals may yield a ratio that falls within the range of ratios indicative of an SWE signal. Such noise signals may, for instance, have a pattern that differs from an SWE event but will have substantially the same pulse frequency which will result in the same pulse count and a pulse count-to-envelope excursion count indicative of an SWE signal.
Thus, there exists a need for a technique that can distinguish between SWE signals and noise signals in substantially all situations, including the situation where the noise has a different pattern than the SWE signal but has substantially the same pulse count.